Exhaust gas filtration device and auxiliary filtration device

ABSTRACT

The present invention has an object to improve the efficiency of collection of solidification constituents and solids in exhaust gas and to prevent early blockage of the filter without damaging the vacuum pump. In an exhaust path 48 a , a vacuum pump and exhaust gas filtration device are provided. This exhaust gas filtration device is constituted by a trap device, pre-filter and filter. The pre-filter reduces the exhaust gas flow rate flowing through the interior of the exhaust path by controlling the exhaust gas flow path in the vessel. The aforesaid exhaust path is constituted by connecting this vacuum pump, trap device, pre-filter and filter which are arranged in this order from the side of airtight vessel and connected through piping if required.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an exhaust gas filtration devicefor removing solidification constituents and solids in exhaust gas,which is provided in the exhaust path of a gas treatment chamberemployed in a step of manufacturing semiconductor elements or electroniccomponents.

[0003] 2. Description of Related Art

[0004] In, for example, plasma CVD equipment which is used in themanufacture of semiconductor elements or electronic components, a plasmaCVD process occurs in an airtight vessel, and an a-Si film or SiN film,etc. is deposited on a substrate. In this process, apart from on thesubstrate, the thin film is deposited on the inside wall etc of theairtight vessel. Usually, the thin film that is deposited on the insidewall etc is removed by plasma cleaning using NF₃ gas. In this process,gaseous products chiefly represented by Si₂F₆(NH₄)₃.F* are produced inthe airtight vessel. Evacuation of the airtight vessel is continuedduring plasma cleaning, so powder-form solids chiefly represented bySi₂F₆(NH₄)₃.F* are precipitated and deposited on the piping, etc.constituting the exhaust path. Such deposition of solids tends to causeblockage of the piping. In order to prevent this, an exhaust gasfiltration device is provided in the exhaust path with the object ofremoving solidification constituents (gaseous products whose conditionis changed to a solid by cooling or by densification (raised pressure))and solids in the exhaust gas.

[0005]FIG. 13 is a block diagram illustrating a conventional exhaust gasfiltration device. FIG. 13(A) shows an airtight vessel 10 and exhaustpath 12 a of this airtight vessel. A vacuum pump 14 and exhaust gasfiltration device 16 are arranged in this exhaust path 12 a. Thisconventional exhaust gas filtration device 16 comprises a trap device 18and filter 20. The exhaust path 12 a referred to above is constituted byconnecting this vacuum pump 14, trap device 18 and filter 20 in thisorder from airtight vessel 10 by means of piping in accordance withrequirements.

[0006] Also, the exhaust path 12 b shown in FIG. 13(B) consists of trapdevice 18, filter 20 and vacuum pump 14 connected in this order throughpiping, as required, from airtight vessel 10. In this way, for the orderof arrangement of the exhaust gas filtration device 16 and vacuum pump14 an order may be employed that is the opposite of that of FIG. 13(A).

[0007] As the trap device 18 described above, for example trap devicesconstructed as shown in FIGS. 14 and 15 are known. FIGS. 14 and 15 arecross-sectional views illustrating the construction of typical trapdevices.

[0008] The trap device shown in FIG. 14 comprises a cylindrical vessel(casing) 22 having apertures at both ends. One aperture of this vessel22 is employed as a gas inlet port 24 and the other aperture of thisvessel 22 is employed as a gas outlet port 26, respectively. Withinvessel 22, there is provided a cylindrical baffle plate 28 which isclosed at one end. Baffle plate 28 is arranged in the vicinity of themiddle of the interior of vessel 22, with its closed end facing gasinlet port 24. Within this baffle plate 28, there is provided a coolingpipe 30 comprising a cooling medium inlet port 30 a and cooling mediumoutlet port 30 b. Also, on the wall surface of vessel 22, there isprovided a cooling pipe 32 comprising a cooling medium inlet port 32 aand cooling medium outlet port 32 b. A cooling medium such as water iscirculated in these cooling pipes 30 and 32.

[0009] Exhaust gas evacuated from the airtight vessel flows from gasinlet port 24 into the interior of vessel 22 and, passing between theinside wall of vessel 22 and baffle plate 28, flows from gas outlet port26 into the downstream exhaust path. This exhaust gas carries heat. Onthe other hand, vessel 22 and baffle plate 28 are cooled to atemperature lower than the temperature of the exhaust gas by means ofcooling pipes 30 and 32. As a result, the exhaust gas solidifies in thevessel 22, and products generated within the airtight vessel areprecipitated as solids. These solids are deposited on the wall surfaceof vessel 22 and the surface of baffle plate 28.

[0010] Also, in the trap device shown in FIG. 15, a cooling pipe 34comprising a cooling medium inlet port 34 a and cooling medium outletport 34 b is provided within vessel 22. This cooling pipe 34 is of ashape that is bent a plurality of times, so the contact area between theexhaust gas and cooling pipe 34 is increased, and the efficiency ofcollection of the solidification constituents and solids is increased.

[0011] Next, a typical example of the construction of the filter 20referred to above is illustrated in FIG. 16. FIG. 16(A) is across-sectional view showing an example of construction of the filter.FIG. 16(B) is a perspective view with part of this filter disassembled.

[0012] The filter shown in FIG. 16 comprises a cylindrical vessel 36having two apertures 38 and 40. The first aperture 38 of this vessel 36is used as a gas inlet aperture and the second aperture 40 of thisvessel 36 is used as a gas outlet aperture, respectively. In thisexample, the second aperture 40 is formed at one end of vessel 36 whilethe first aperture 38 is formed in the cylindrical surface nearer to theother end of vessel 36.

[0013] A filter mesh 42 is provided in the interior of vessel 36. Thisfilter mesh 42 is constituted by winding a stainless steel plain fabricdiamond wire diameter mesh (hereinbelow abbreviated to “mesh”) 44 ontothe outside of a stainless-steel cylindrical frame 46 (in a condition inwhich frame 46 is inserted facing in the direction shown by arrow a inFIG. 16(B)). This filter mesh 42 is arranged such that its inside (onthe side of frame 46) communicates with second aperture 40 and itsoutside (on the side of mesh 44) communicates with first aperture 38. Aplurality of apertures 46 a are formed on the cylindrical surface offrame 46 so that exhaust gas that flows into the first aperture 38passes through the mesh 44 of filter mesh 42 and reaches the secondaperture 40. Solids in the exhaust gas are captured by mesh 44.

[0014] Also, first aperture 38 could be used as a gas outlet port andsecond aperture 40 could be used as a gas inlet port. In this case, theexhaust gas flows into the second aperture 40, and the exhaust gaspasses through the mesh 44 of filter mesh 42 before flowing out to theoutside from first aperture 38.

[0015] However, the conventional exhaust gas filtration devices sufferfrom the following problems.

[0016] 1) In the trap devices described above, the solidificationconstituents or solids in the exhaust gas cannot, be completely removed.In order to remove the solidification constituents or solids, it isnecessary to cool the exhaust gas, thereby inevitably bringing thesolidification constituents or solids in the exhaust gas into contactwith cooling parts of the trap device. It is therefore difficult toremove fine particulate products (solids) that do not flow through thevicinity of the cooling parts. Also, even if they do come into contact,it is difficult for fine particulate products that are flowing past withhigh speed to be deposited and accumulated.

[0017] 2) The products described above that are not captured by the trapdevice are removed by a filter provided downstream of the trap device.However, although this filter is able to remove the fine particulateproducts due to the fact that it consists of fibrous members of a fineclose construction, it is easily blocked even by a very small quantityof particulate products, severely lowering the conductance of theexhaust path. When the conductance has been lowered to a certain degree,it is necessary to wash or change the structural components of theexhaust gas filtration device. Consequently, due to the employment of afilter, the period of continuous use of the exhaust gas filtrationdevice and the semiconductor manufacturing equipment employing isshortened.

[0018] 3) In order to solve the problem of 2) above, the amount of fineparticulate products flowing into the filter must be reduced. Notingthat the collection efficiency of the trap device is inverselyproportional to the flow velocity of the exhaust gas flowing through theinterior, it might be considered that it would be effective todeliberately reduce the conductance at an arbitrary position within theexhaust path. However, if the conductance is lowered to such a level asto solve the problem of 2) above, the load applied to the vacuum pumpbecomes large, giving rise to the fresh problem of damage to the vacuumpump.

[0019] 4) Also, in the conventional trap devices, there was localaccumulation of solids on the cooling pipes in the vicinity of the gasinlet aperture but scarcely any accumulation of the solids was found oncooling pipes remote from the gas inlet aperture. Consequently,reduction in the conductance of the gas flow path due to accumulation ofsolids proceeds locally in the vicinity of the inlet port. Thistherefore shortens the time of use until the trap device must be changedor washed, and means that satisfactory performance in regard tocollection efficiency is not achieved.

[0020] The object of the invention of this application is therefore toprovide an exhaust gas filtration device and auxiliary filtration deviceaiming at improving the collection efficiency of solidificationconstituents and solids in exhaust gas, yet in which the period ofcontinuous use can be extended without damaging the vacuum pump.

[0021] A further object of this application is to provide a trap devicein which the period of use of the device can be extended and thecollection efficiency of solids can be improved by promoting depositionof solids at locations other than the vicinity of the gas inlet port.

SUMMARY OF THE INVENTION

[0022] In order to achieve this object, an exhaust gas filtration deviceaccording to the present invention comprising a trap device and a filterarranged successively in an exhaust path of an airtight vessel evacuatedby a vacuum pump, for removing solidification constituents and solids inthe exhaust gas evacuated into this exhaust path, further comprises: anauxiliary filtration device arranged in the exhaust path between thetrap device and the filter.

[0023] By thus providing an auxiliary filtration device, some of thesolids which were difficult to collect and accumulate by the trap deviceare removed by this auxiliary filtration device upstream of the filter.Consequently, early blockage of the filter can be prevented, enablingthe life of the exhaust gas filtration device as a whole to be extended.

[0024] Also, by the provision of an auxiliary filtration device, theconductance of the exhaust path can be deliberately reduced to an extentsuch that the vacuum pump is not damaged. As a result, the flow velocityof exhaust gas within the trap device is lowered i.e. the dwell time ofexhaust gas within the trap device is extended, and the collectionefficiency for solidification constituents and solids in the trap deviceis improved.

[0025] Also, according to the invention of the auxiliary filtrationdevice, there are provided a vessel, exhaust gas inlet pipe, exhaust gasoutlet pipe and filter element constituting a device for removal ofsolids in exhaust gas discharged into an exhaust path and arranged inthis exhaust path of an airtight vessel evacuated by a vacuum pump.

[0026] According to the present invention, this filter element is asponge-like aggregate constituted by collecting a large number ofstrip-shaped or filamentous members.

[0027] Also, according to the present invention, the interior of thevessel and the exhaust path are connected by an exhaust gas inlet pipeand exhaust gas outlet pipe.

[0028] Furthermore, according to the present invention, a filtrationregion, a first diffusion region and second diffusion region are definedin the interior of the vessel, and the first and second diffusionregions are separated from each other by a filter element arranged inthe filtration region.

[0029] Furthermore, according to the present invention, the end of theexhaust gas inlet pipe constitutes a gas inlet port and is arranged ineither the first or second diffusion region, and the end of the exhaustgas outlet pipe constitutes a gas outlet port and is arranged in eitherthe first or second diffusion region.

[0030] Furthermore, according to the present invention, the exhaust gasflow path extending from the gas inlet port to the gas outlet port isconstructed such that exhaust gas passes through the filter element atleast once.

[0031] Usually, this auxiliary filtration device is arranged between thetrap device and the filter. The filter element that is installed in thisauxiliary filtration device is a member that captures principallycomparatively large fine products (solids) that are not removed by thetrap device and that cause blockage of the filter.

[0032] With such an auxiliary filtration device, the exhaust gas flowsinto the first or second diffusion region through the exhaust gas inletpipe. After this, the exhaust gas passes through the filter element andreaches the gas outlet port arranged in the first or second diffusionregion. The exhaust gas is then fed to the downstream waste path by theexhaust gas outlet pipe. In this process, solids in the exhaust gas areremoved.

[0033] Also, thanks to the provision of the filter element, theconductance of the exhaust path at the position where the filter elementis arranged is lowered. The flow velocity of the exhaust gas flowingthrough the exhaust path upstream of the position where the conductanceis lowered is therefore reduced. As a result, the efficiency ofcollection of solidification constituents and solids in the otherfiltration device arranged upstream of this auxiliary filtration deviceis improved.

[0034] Preferably in the auxiliary filtration device of the presentinvention, part of the through-flow path of the exhaust gas constitutesa path whereby exhaust gas flows in the opposite direction to thedirection in which exhaust gas is evacuated from the airtight vessel.

[0035] With such an arrangement, the direction of through-flow of theexhaust gas from the gas inlet port to the gas outlet port ispractically opposite to the direction of inflow of exhaust gas into thevessel from the gas inlet port and to the direction of outflow ofexhaust gas to the gas outlet port from the interior of the vessel. Theflow velocity of the exhaust gas flowing through the interior of theexhaust path upstream of the auxiliary filtration device is therebyfurther reduced, as a result of which the efficiency of collection ofsolidification constituents and solids in the other filtration devicearranged upstream of this auxiliary filtration device can be expected tobe further improved.

[0036] Also, in a preferred example of the auxiliary filtration deviceof the present invention, it is preferable that the gas inlet port isarranged in the first diffusion region and the gas outlet port isarranged in the second diffusion region, the exhaust gas inlet pipebeing coupled to the exhaust path through a partition on the side of thesecond diffusion region of the vessel, while the exhaust gas outlet pipeis coupled to the exhaust path through a partition on the side of thefirst diffusion region of the vessel.

[0037] With this arrangement, a portion of the exhaust gas through-flowpath becomes a path whereby the exhaust gas flows in the oppositedirection to the direction whereby the exhaust gas is evacuated from theairtight vessel.

[0038] Also, in another preferred example of the auxiliary filtrationdevice of the present invention, it is preferable that a partition thatdivides the first diffusion region into an exhaust gas inlet region andan exhaust gas outlet region, and the filtration region into two,namely, a first and second filtration region, is provided in the vessel;the gas inlet port is arranged in the exhaust gas inlet region and thegas outlet port is arranged in the exhaust gas outlet region; theexhaust gas inlet pipe is coupled with the exhaust path through apartition on the side of the second diffusion region of the vessel; andthe exhaust gas outlet pipe is coupled with the exhaust path through apartition on the side of the first diffusion region of the vessel.

[0039] With this arrangement, a portion of the exhaust gas through-flowpath becomes a path whereby the exhaust gas flows in the oppositedirection to the direction whereby the exhaust gas is evacuated from theairtight vessel.

[0040] Also, preferably, in the auxiliary filtration device of thepresent invention, the filter element is constituted by a plurality ofmetal strips which are packed substantially uniformly between aplurality of support plates having at least one aperture.

[0041] Furthermore, in implementation of the auxiliary filtration deviceof the present invention, preferably a cooling mechanism is provided forcooling the filter element.

[0042] Also, it has been noted that generation of solids depends also onpressure, not solely on temperature.

[0043] Specifically, in a trap device according to the inventionrelating to the present embodiment, a trap device arranged on an exhaustpath of an airtight vessel evacuated by a vacuum pump, for removingsolidified gas as solid in this exhaust path, is constituted by a vesselhaving in its interior a gas flow path connected to the exhaust path,the flow velocity of gas in the flow path being controlled to a certainflow rate in accordance with position on this flow path.

[0044] In this way, the flow velocity of gas is controlled in accordancewith flow path position, so accumulation of solids is promoted where theflow velocity is comparatively small. On the other hand, accumulation ofsolids is avoided where the flow velocity is comparatively large. It istherefore possible to cause solids to be accumulated at prescribedpositions on the flow path and to prevent accumulation of solids atlocations where lowering of the conductance of the flow path is notdesired. Accumulation of solids can thereby be promoted in locationsother than the vicinity of the gas inlet port, thereby enabling theperiod of use of the device to be extended and also improving theefficiency of collection of solids.

[0045] In a preferred example of the trap device of the presentinvention, the flow path comprises a main flow path extending in helicalfashion and an auxiliary flow path branched from part of this main flowpath and connected to another part of this main flow path.

[0046] With such a construction, the gas flowing in the main flow pathis slowed down by the gas flowing in from the auxiliary flow path atpoints where the main flow path and auxiliary flow path merge. The dwelltime of the gas in the device is thereby extended and accumulation ofsolids is promoted. Also, accumulation of solids in the main flow pathis promoted as the period of use of the device increases, causing thecross-sectional area of the flow path to be reduced, but, since the gasflows into the downstream part of the main flow path through theauxiliary flow path, the downstream part of the main flow path can alsobe effectively utilized. Consequently, the period of use of the devicecan be extended compared with conventionally.

[0047] Also, in a trap device according to the present invention,preferably, the aforementioned main flow path is formed by a thin plateconnected to the surface of a shaft element provided in the interior ofthe vessel, and the auxiliary flow path is formed by an aperture formedat a prescribed position of the thin plate.

[0048] Also, in another preferred example of the trap device of thepresent invention, the flow path comprises a plurality of annular firstflow paths and second flow paths connected between the first flow paths,and the flow path cross-sectional area of the first flow paths ischanged at prescribed positions.

[0049] If such a construction is adopted, the gas flows in each of thefirst flow paths and second flow paths. The gas flowing into the firstflow paths from the second flow paths is branched into two streams. Dueto the provision of prescribed locations where the flow pathcross-sectional area changes in the first flow paths, the gas proceedsthrough respective locations where the flow path cross-sectional area issmall and where the flow path cross-sectional area is large. Inlocations where the flow path cross-sectional area is small, the gasflow velocity becomes faster than where the flow path cross-sectionalarea is larger. Consequently, it is more difficult for solids toaccumulate in the locations where the flow path cross-sectional area issmall, while, on the other hand, solids accumulate more easily where theflow path cross-sectional area is large. Thus, locations whereaccumulation of solids is promoted and locations where lowering of theconductance is prevented can be set up at prescribed positions.Consequently, it is possible to induce non-local accumulation of solids,so that the downstream sections of the first flow paths are alsoeffectively used. Consequently, the period of use of the device islonger than conventionally.

[0050] Also, preferably, in a trap device according to the presentinvention, the first flow path is formed by a plurality of thin platesconnected to the surface of a shaft element arranged in the interior ofthe vessel, the second flow path is formed by apertures formed inprescribed positions of the thin plates, and the flow pathcross-sectional area of the first flow paths is changed by forming astep at a prescribed position of the thin plates.

[0051] Also, preferably, the thin plates are bent in irregular orundulating fashion. This is because the surface area of the thin platesis thereby increased, increasing the effective area on which solids canbe accumulated. Also, the gas flow path is extended, enabling the periodof use of the device to be extended and the collection efficiency ofsolids to be improved.

[0052] Furthermore, preferably, an irregular structure is formed in thesurface of said thin plates. In order to achieve this, for example, anirregular surface may suitably be formed by subjecting the surface ofthe thin plates to blast processing. As a result, the surface area ofthe thin plates is increased.

[0053] Furthermore, suitably, a cooling mechanism may be provided in theinterior of the shaft element referred to above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0054]FIG. 1 is a view showing the construction of an exhaust gasfiltration device according to an embodiment;

[0055]FIG. 2 is a view showing the construction of a pre-filteraccording to a first embodiment;

[0056]FIG. 3 is a view showing the construction of a pre-filteraccording to a first embodiment;

[0057]FIG. 4 is a view showing the construction of a support plate;

[0058]FIG. 5 is a view showing the construction of a pre-filteraccording to a second embodiment;

[0059]FIG. 6 is a view showing the construction of a trap deviceaccording to a third embodiment;

[0060]FIG. 7 is a view showing the construction of a cooling mechanism;

[0061]FIG. 8 is a view provided to describe the operation of the trapdevice of the third embodiment;

[0062]FIG. 9 is a view showing a modification of the trap device of thethird embodiment;

[0063]FIG. 10 is a view showing the construction of a trap deviceaccording to a fourth embodiment;

[0064]FIG. 11 is a view provided to describe the operation of the trapdevice of the fourth embodiment;

[0065]FIG. 12 is a view illustrating a modification of the trap deviceof the fourth embodiment;

[0066]FIG. 13 is a view showing a conventional exhaust gas filtrationdevice;

[0067]FIG. 14 is a view showing the construction of a typical trapdevice;

[0068]FIG. 15 is a view showing the construction of a typical trapdevice; and

[0069]FIG. 16 is a view showing the construction of a typical filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0070] Embodiments of the invention are described below with referenceto the drawings. The drawings illustrate schematically the shape,dimensions and arrangement relationships of the various structuralcomponents in order to enable the present invention to be understood. Inthe drawings used in the description below, the same reference symbolsare attached to similar structural components. The numerical valueconditions and materials set out below merely constitute one example.Accordingly, the present invention is not restricted in any way to theseembodiments.

[0071] The exhaust gas filtration device of this embodiment is a devicearranged in the exhaust path of an airtight vessel that is evacuated bya vacuum pump for removal of the solidification constituents (gaseousproducts) and solids in the exhaust gas discharged into this exhaustpath.

[0072]FIG. 1 is a block diagram illustrating the construction of theexhaust gas filtration device of the embodiment.

[0073] In FIG. 1(A), there are shown an airtight vessel 10 and anexhaust path 48 a of this airtight vessel. In this exhaust path 48 athere are provided a vacuum pump 14 and exhaust gas filtration device 50of this embodiment. This exhaust gas filtration device 50 comprises atrap device 18, pre-filter (auxiliary filtration device. Also called apre-filtration device) 52 and a filter 20. A special feature of thisexhaust gas filtration device 50 is that a pre-filter 52 is newlyprovided between the trap device and filter constituting a conventionalexhaust gas filtration device. The exhaust path 48 a referred to aboveis constituted by connecting this vacuum pump 14, trap device 18,pre-filter 52 and filter 20 in this order from the airtight vessel 10through piping if required.

[0074] Also, the exhaust path 48 b shown in FIG. 1(B) is constituted byconnecting exhaust gas filtration device 50 and vacuum pump 14 in thisorder from the airtight vessel 10 through piping if required. In thisway, the order of arrangement of exhaust gas filtration device 50 andvacuum pump 14 is the opposite to that in which they are used in thecase of FIG. 1(A).

[0075] Further, for the exhaust path 48 c shown in FIG. 1 (C), vacuumpump 14 a, exhaust gas filtration device 50 and vacuum pump 14 b areconnected in this order from airtight vessel 10 through piping ifrequired. Thus, vacuum pumps may be provided both upstream anddownstream of exhaust gas filtration device 50. In general, as theupstream vacuum pump 14 a, a pump suited for creating a medium vacuum orhigh vacuum condition, such as, for example, a turbo-molecular pump, isemployed, whereas for the downstream vacuum pump 14 b, a pump suited forcreating a vacuum condition from atmospheric pressure, such as forexample a dry pump is employed.

[0076] As the trap device 18 described above, for example a trap deviceas described with reference to FIGS. 14 and 15 may be employed. Apartfrom this, trap devices according to the third and fourth embodiment, tobe described, may be employed. Also, as filter 20 described above, forexample the filter described with reference to FIG. 16 may be employed.

[0077] First Embodiment

[0078] Next, a first embodiment of pre-filter 52 described above will bedescribed with reference to FIGS. 2, 3 and 4. FIG. 2 is across-sectional view illustrating the construction of a pre-filteraccording to a first embodiment. The cross-sectional view shown in FIG.3 illustrates a cross section cut perpendicularly with respect to thedirection of extension of exhaust gas inlet pipe 56 and exhaust gasoutlet pipe 58 of the portion of the filtration region 62 of thepre-filter shown in FIG. 2. Also, FIG. 4 is a plan view illustrating theconstruction of support plate 72 shown in FIG. 2.

[0079] As shown in FIG. 2, this pre-filter chiefly comprises vessel 54,exhaust gas inlet pipe 56, exhaust gas outlet pipe 58 and filter element60.

[0080] Vessel 54 is a cylindrically shaped vessel having one aperture ateach of its two ends, respectively. An exhaust gas inlet pipe 56 isinserted in one aperture of this vessel 54, while an exhaust gas outletpipe 58 is inserted in the other aperture. The interior of vessel 54 andthe exhaust path are connected by means of this exhaust gas inlet pipe56 and exhaust gas outlet pipe 58. Prescribed sealing is effected in theregion of the apertures of vessel 54 through which this exhaust gasinlet pipe 56 and exhaust gas outlet pipe 58 pass, in order to maintaingas tightness within vessel 54.

[0081] A filtration region 62, first diffusion region 64 and seconddiffusion region 66 are defined in the interior of vessel 54. This firstand second diffusion regions 64 and 66 are isolated by means of a filterelement 60 provided in filtration region 62. This filter element 60 isconstituted by a sponge-like assembly in which a large number of membersof tape or filament form are collected together. Such a filter element60 has the function of capturing solids in exhaust gas. This filterelement 60 is supported by a plurality of support pillars 68 in thevicinity of the center of vessel 54.

[0082] Exhaust gas inlet pipe 56 and exhaust gas outlet pipe 58 referredto above are respectively arranged so as to extend linearly withinvessel 54. Also, exhaust gas inlet pipe 56 is coupled to the exhaustpath through a partition on the side of the second diffusion region 66of the vessel 54 and exhaust gas outlet pipe 58 is coupled to theexhaust path through a partition on the side of the first diffusionregion 64 of vessel 54.

[0083] The end of the exhaust gas inlet pipe 56 mentioned above is gasinlet port 56 a and is arranged within the first diffusion region 64.Also, the end of exhaust gas outlet pipe 58 referred to above is a gasoutlet port 58 a and is arranged within second diffusion region 66.

[0084] Consequently, the through-flow path of exhaust gas from gas inletport 56 a to gas outlet port 58 a is constituted such that the exhaustgas passes at least once through filter element 60.

[0085] Furthermore, part of this exhaust gas through-flow path, inparticular the path when the exhaust gas passes through filter element60, constitutes a path whereby the exhaust gas flows in a directionopposite to the direction in which the exhaust gas is evacuated fromairtight vessel 10. That is, the direction whereby the exhaust gas flowsthrough from gas inlet port 56 a to gas outlet port 58 a is a directionpractically opposite to the direction of the inflow of the exhaust gasfrom gas inlet port 56 a into vessel 54 and to the direction of exhaustgas outflow from vessel 54 to gas outlet port 58 a.

[0086] Next, a specific construction of filter element 60 referred toabove will be described. The filter element 60 of this example isconstituted by packing a plurality of metal strips between a pluralityof support plates 72 having apertures substantially uniform such thatexhaust gas can flow therethrough. In the example shown in FIGS. 2 and3, metal wool 70 constituting an assembly of metal strips is packed inthe space between two support plates 72 of the same shape.

[0087] The metal strips constituting metal wool 70 are obtained bycutting processing of metal such as stainless steel into the form ofstrips of arbitrary length, width 2 mm and thickness about 0.1 mm. Suchmetal wool 70 has a sponge-like structure like a metal brush. The shapeof the metal strips is not restricted to the example described above,but could be of any desired shape. For example, they could be made offilamentous shape or could be wound in helical fashion. Furthermore,glass strips could be used instead of the metal strips.

[0088] Exhaust gas can flow through the interior of such metal wool 70and the contact area between the metal wool 70 and the exhaust gas isextremely large. Consequently, any solids in the exhaust gas that havenot been completely removed in the trap device can be removed by captureby the metal wool 70.

[0089] The support plates 72 consist of disc shaped stainless steelplates. A plurality of through-holes are formed in support plate 72. Inthe middle of the disc of a support plate 72, there is formed a circularinlet aperture port 74 through which exhaust gas inlet pipe 56 ispassed. Also, in a support plate 72 there is formed a circular outletaperture port 76 through which exhaust gas outlet pipe 58 passes. Also,in support plates 72, there are formed a plurality, in this example,four, of support pillar apertures 78 through which are inserted supportpillars 68 for supporting filter element 60. Furthermore, a large numberof through-flow apertures 80 are formed in support plates 72 to allowpassage of exhaust gas.

[0090] The external diameter of these support plates 72 (disc diameter)is equal to the internal diameter of vessel 54. The two support plates72 are arranged parallel to each other fitted into the interior ofvessel 54. Exhaust gas inlet pipe 56, exhaust gas outlet pipe 58 andsupport pillars 68 are respectively inserted into the apertures 74, 76and 78 of the support plates 72 described above in a condition with thesupport plates 72 arranged in vessel 54. Furthermore, the metal wool 70described above is packed between the exhaust gas inlet pipe 56, exhaustgas outlet pipe 58 and supporting pillars 68 between the two supportplates 72 as shown in FIG. 3 (however, supporting pillars 68 are notshown in FIG. 3).

[0091] Also, in this pre-filter, with the object of obtaining aprecipitation effect of the solidification constituents in filterelement 60, a cooling mechanism for cooling filter element 60 i.e.support plates 72 and metal wool 70 is provided. As this coolingmechanism, a cooling pipe 82 for circulation of cooling medium such aswater is employed. The construction is such that cooling pipe 82 isarranged at the outside surface of vessel 54, filter element 60 beingcooled through the partition of vessel 54.

[0092] Next, the operation of this pre-filter will be described.

[0093] First of all, exhaust gas evacuated from the trap device flowsinto exhaust gas inlet pipe 56. The exhaust gas flows into firstdiffusion region 64 within vessel 54 through gas inlet port 56 a ofexhaust gas inlet pipe 56. The direction of inflow of the exhaust gas isthen a direction away from filtration region 62 in vessel 54.

[0094] Next, exhaust gas is diffused in first diffusion region 64. Thediffused exhaust gas flows in the opposite direction to the direction ofinflow and arrives at the second diffusion region 66 by passing throughthe interior of the filter element 60 arranged in filtration region 62.In this process, solidification constituents and solids in the exhaustgas are removed in filter element 60.

[0095] Next, the exhaust gas in the second diffusion region 66 isallowed to flow to the outside of vessel 54 by means of exhaust gasoutlet pipe 58, passing through gas outlet port 58 a.

[0096] Thus, with this pre-filter, since filter element 60 is arrangedin the through-flow path of the exhaust gas, the flow velocity of theexhaust gas flowing through the interior of the trap device upstream ofthis pre-filter is lowered and as a result the dwell time of the exhaustgas in the trap device is prolonged. Consequently, the collectionefficiency for solidification constituents and solids in the trap deviceis improved.

[0097] Furthermore, with this pre-filter, the direction in which theexhaust gas flows through the filter element 60 is controlled in adirection practically opposite to the direction in which the exhaust gasflows from gas inlet port 56 a into vessel 54 and the direction in whichthe exhaust gas flows out from the interior of vessel 54 into gas outletport 58 a. As a result, the flow velocity of the exhaust gas flowingthrough the interior of the trap device upstream of the pre-filter isfurther reduced, so the efficiency of collection of solidificationconstituents and solids in the trap device is further improved.

[0098] Second Embodiment

[0099] Next, a second constructional example of pre-filter 52 referredto above will be described with reference to FIG. 5. FIG. 5 is across-sectional view showing the construction of a pre-filter accordingto a second embodiment. Hereinbelow, the description will beconcentrated on those aspects of this second constructional examplewhich are different from the first constructional example.

[0100] In the pre-filter of this constructional example, a partition 84for separating the first diffusion region 64 into an exhaust gas inletregion 64 a and exhaust gas outlet region 64 b is provided in vessel 54.This partition 84 is arranged between this exhaust gas inlet pipe 56 andexhaust gas outlet pipe 58 in a condition parallel to exhaust gas inletpipe 56 and exhaust gas outlet pipe 58.

[0101] Gas inlet port 56 a is arranged in exhaust gas inlet region 64 aand gas outlet port 58 a is arranged in exhaust gas outlet region 64 b.

[0102] Also, the filtration region 62 is divided by the aforementionedpartition 84 into two, namely, first filtration region 62 a and secondfiltration region 62 b. Exhaust gas inlet pipe 56 is arranged so as toextend within the first filtration region 62 a on the side of theexhaust gas inlet region 64 a.

[0103] Consequently, the through-flow path of exhaust gas from gas inletport 56 a to gas outlet port 58 a is constituted such that the exhaustgas passes at least once through filter element 60.

[0104] Furthermore, part of this exhaust gas through-flow path, inparticular the path when the exhaust gas passes through first filtrationregion 62 a, constitutes a path whereby the exhaust gas flows in adirection opposite to the direction in which the exhaust gas isevacuated from airtight vessel 10. That is, the direction whereby theexhaust gas flows through from gas inlet port 56 a to gas outlet port 58a is a direction practically opposite to the direction of the inflow ofthe exhaust gas from gas inlet port 56 a into vessel 54 and to thedirection of exhaust gas outflow from vessel 54 to gas outlet port 58 a.

[0105] Next, the operation of this pre-filter will be described.

[0106] First of all, exhaust gas evacuated from the trap device flowsinto exhaust gas inlet pipe 56. The exhaust gas flows into exhaust gasinlet region 64 a within vessel 54 through gas inlet port 56 a ofexhaust gas inlet pipe 56. The direction of inflow of the exhaust gas isthen a direction away from first filtration region 62 a in vessel 54.

[0107] Next, exhaust gas is diffused in exhaust gas inlet region 64 a.The diffused exhaust gas flows in the opposite direction to thedirection of inflow and arrives at the second diffusion region 66 bypassing through the interior of the filter element 60 arranged in firstfiltration region 62 a. In this process, solidification constituents andsolids in the exhaust gas are removed in filter element 60.

[0108] Next, the exhaust gas in the second diffusion region 66 isallowed to flow to exhaust gas outlet region 64 b by passing through thefilter element 60 arranged in second filtration region 62 b. In thisprocess, solidification constituents and solids in the exhaust gas arefurther removed in filter element 60.

[0109] Next, the exhaust gas in the exhaust gas outlet region 64 b isallowed to flow to the outside of vessel 54 by means of exhaust gasoutlet pipe 58, passing through gas outlet port 58 a.

[0110] Thus, with this pre-filter, since filter element 60 is arrangedin the through-flow path of the exhaust gas, the flow velocity of theexhaust gas flowing through the interior of the trap device upstream ofthis pre-filter is lowered and as a result the dwell time of the exhaustgas in the trap device is prolonged. Consequently, the collectionefficiency for solidification constituents and solids in the trap deviceis improved.

[0111] Furthermore, with this pre-filter, the direction in which theexhaust gas flows through the filter element 60 arranged in the firstfiltration region 62 a is controlled in a direction practically oppositeto the direction in which the exhaust gas flows from gas inlet port 56 ainto vessel 54 and the direction in which the exhaust gas flows out fromthe interior of vessel 54 into gas outlet port 58 a. As a result, theflow velocity of the exhaust gas flowing through the interior of thetrap device is further reduced, so the efficiency of collection ofsolidification constituents and solids in the trap device is furtherimproved.

[0112] It should be noted that, although the pre-filter of the secondconstructional example was used in a condition with the exhaust gasinlet pipe 56 coupled with the exhaust path on the side of the trapdevice and exhaust gas outlet pipe 58 coupled with the exhaust path onthe filter side, it could be used with these reversed. Specifically,exhaust gas inlet pipe 56 could be coupled with the exhaust path on thefilter side, while exhaust gas outlet pipe 58 is coupled with theexhaust path on the trap device side.

[0113] In the exhaust gas filtration device described above, a trapdevice is employed together with a pre-filter. Consequently, thepre-filter may suitably be incorporated on the exhaust gas port side ofthe trap device.

[0114] Depending on the conditions of the process such as CVD performedin the airtight vessel (in particular on the type of gas employed and/orthe set temperature), in some cases, solidification constituents may notbe generated, only solids being produced (although solidificationconstituents may be generated, they immediately change in state tosolids). In this case, an exhaust gas filtration device may beconstituted consisting solely of a pre-filter and filter, without usinga trap device.

[0115] Third Embodiment

[0116] Next, a trap device according to a third embodiment will bedescribed. FIG. 6 is a view showing the construction of the trap deviceaccording to the third embodiment. FIG. 6(A) shows a perspective view ofthe trap device and FIG. 6(B) shows a side view of the trap device.

[0117] The trap device shown in FIG. 6 is constituted by a vessel(casing) 86 of cylindrical shape provided with apertures at both ends.One aperture of vessel 86 is employed as a gas inlet port 88, while theother aperture of vessel 86 is employed as a gas outlet port 90. Thisgas inlet port 88 and gas outlet port 90 are respectively connected tothe exhaust path. Also, a flow path of gas (gas mixture) connected tothe exhaust path is formed between gas inlet port 88 and gas outlet port90 within vessel 86. This trap device is arranged in the exhaust pathdescribed above such that gas inlet port 88, the flow path and gasoutlet port 90 are arranged in the horizontal direction.

[0118] In the trap device of this embodiment, the flow velocity of thegas in the flow path described above is controlled to a certain flowvelocity in accordance with the position of this flow path. In order toachieve this, in the third embodiment, the flow path described above isconstituted by a main flow path extending in helical fashion and anauxiliary flow path branched from part of this main flow path andconnected to another portion of this main flow path. In the thirdembodiment, the main flow path described above is formed by a thin plate94. This thin plate 94 is connected to the surface of a cylindricallyshaped shaft element 92 provided within vessel 86. Also, the auxiliaryflow path mentioned above is formed by apertures 96 (not shown in FIG.6(B)) formed at a prescribed positions of thin plate 94.

[0119] Shaft element 92 referred to above is arranged at a position suchthat the central axis of this shaft element 92 coincides with thecentral axis of vessel 86. One end of shaft element 92 is connected to awall portion on the side of gas inlet port 88 of vessel 86, while theother end of shaft element 92 is connected to a wall portion on the gasoutlet port 90 side of vessel 86. Gas inlet port 88 and gas outlet port90 respectively communicate with the interior of shaft element 92. Also,apertures 98 and 100 are respectively formed in the wall face in thevicinity of both ends of shaft element 92. Consequently, these apertures98 and 100 respectively communicate with gas inlet port 88 and gasoutlet port 90. It should be noted that, although not shown in FIG. 6, acooling mechanism 102 is provided between apertures 98 and 100 withinshaft element 92. The interior of shaft element 92 between apertures 98and 100 is blocked by this cooling mechanism 102. It is thereforeimpossible for gas entering gas inlet port 88 to be fed to gas outletport 90 by flowing through the interior of shaft element 92. That is,gas entering gas inlet port 88 is fed into the interior of vessel 86through aperture 98 on the side of gas inlet port 88, passes through theflow path in vessel 88 and is fed to the exhaust path from gas outletport 90, passing through the aperture 100 on the side of gas outlet port90.

[0120] Also, the thin sheet 94 referred to above is a single sheetextending in helical shape centered on shaft element 92 from the side ofgas inlet port 88 to the side of gas outlet port 90. The space betweenthe adjacent parts of this thin plate 94 is employed as the main flowpath referred to above. Gas that is fed into vessel 86 from the side ofgas inlet port 88 flows in helical fashion centered on shaft element 92along the main flow path towards gas outlet port 90.

[0121] Also, as described above, auxiliary flow paths are formed byapertures 96 formed in prescribed positions of thin plate 94. That is,these auxiliary flow paths constitute flow paths that are branched frompart of the main flow path and connected to another portion of this mainflow path. Gas that is introduced into vessel 86 from gas inlet port 88flows along the main flow path towards the gas outlet port 90 and partof the gas becomes a branched flow into the auxiliary flow paths,flowing along the axial direction of shaft element 92.

[0122] Cooling mechanism 102 referred to above will now be describedwith reference to FIG. 7. FIG. 7 is a cross-sectional view showing theconstruction of the cooling mechanism. Hatching indicating the crosssection is not shown in the Figure. Also, the cross-sectional planeshown in FIG. 7 is a cross-sectional plane corresponding to a cut at aposition including the central axis of shaft element 92 shown in FIG.6(B) (position of the line I-I of FIG. 6(B)).

[0123] As shown in FIG. 7(A), the cooling mechanism 102 referred toabove is constituted of a cylindrical shaped rod-shaped member 104. Thisrod-shaped member 104 is formed fitting into the interior of shaftelement 92, being formed with respective cut-away portions 104 a and 104b at both its ends. These cut-away portions 104 a and 104 b are formedsuch that, when rod-shaped member 104 is fitted into the interior ofshaft element 92, apertures 98 and 100 of shaft element 92 arerespectively aligned with the positions of cut-away portions 104 a and104 b. Consequently, even though rod-shaped member 104 is inserted intothe interior of shaft element 92, respectively connected conditions areproduced between gas inlet port 88 and aperture 98 and between gasoutlet port 90 and aperture 100. The positions of apertures 98 and 100when rod-shaped member 104 is fitted into the interior of shaft element92 are respectively indicated by broken lines a and b in FIG. 7.

[0124] Also, a cooling pipe 106 is embedded in the interior of therod-shaped member 104. Both end portions of this cooling pipe 106 areled to the outside of rod-shaped member 104 and are respectivelyemployed as cooling medium inlet port 106 a and cooling medium outletport 106 b. The direction of elongation of rod-shaped member 104arranged in shaft element 92 coincides with the horizontal direction andit is arranged in a condition with cooling medium inlet port 106 a andcooling medium outlet port 106 b disposed in the vertical direction.Also, in this condition, cooling medium outlet port 106 b is arranged atthe top, while cooling medium inlet port 106 a is arranged at thebottom. A cooling medium such as water is made to flow through thiscooling pipe 106. Rod-shaped member 104 is thereby cooled, so that shaftelement 92 which is in contact with this member 104 is cooled.Furthermore, thin plate 94 that is connected to this shaft element 92 iscooled, so that the temperature of thin plate 94 is lower than thetemperature of the gas, which is thereby adjusted to a temperaturesuited to the solidification of the gas.

[0125] Although in the examples described above the shaft element 92 andcooling mechanism 102 were separate, an integral construction of thisshaft element 92 and cooling mechanism 102 could be adopted. Forexample, as shown in FIG. 7(B), instead of shaft element 92 describedabove, a shaft element 92 a constituted by a solid rod-shaped memberwith a cooling pipe 106 embedded therein could be employed. At both endsof shaft element 92 a shown in FIG. 7(B) there are formed hollowsections 108 a and 108 b extending along the axial direction of shaftelement 92 a, respectively, these hollow sections 108 a and 108 brespectively communicating with gas inlet port 88 and gas outlet port90. Also, an aperture is formed in the wall surface of shaft element 92a communicating with hollow sections 108 a and 108 b, the aforementionedhollow sections 108 a and 108 b thereby respectively communicating withthe interior of vessel 86. If such a shaft element 92 a is arranged inthe interior of the vessel 86, a construction similar to the exampledescribed above in which shaft element 92 and cooling mechanism 102 areseparately constructed is realized.

[0126] Next, the operation of a trap device constructed as describedabove will be described with reference to FIG. 8. FIG. 8 is a viewprovided to explain the operation of the trap device of the thirdembodiment. FIG. 8 shows the flow path within vessel 86 described above.The two line sections extending in the horizontal direction in the upperand lower part of the Figure indicate vessel 86; thin plate 94 isindicated by the plurality of line sections arranged parallel with eachother between these line sections. Other structural components such asshaft element 92 are omitted from the drawing. The left side in theFigure is the gas inlet port 88 side; the right side in the Figure isthe gas outlet port 90 side. In addition, the arrow symbols in thedrawing indicate the direction of flow of the gas.

[0127] Gas that is introduced into vessel 86 passes along the flow pathand flows from the gas inlet port 88 side towards the gas outlet port 90side i.e. from the left-hand side in the Figure towards the right-handside in the Figure. The gas flowing along the main flow path passes inthis order through the vicinities of the locations indicated by thesymbols a, b, c, d, e, f, g and h in the drawing. Some of the gasflowing along the main flow path passes through the apertures 96 formedin thin plate 94 i.e. the auxiliary flow paths, and flows out into otherportions of the main flow path. Some of the gas flowing out from theseauxiliary flow paths slows the flow of gas in the main flow path. Thedwell time of the gas in the vicinity of the region where the auxiliaryflow path and main flow path merge is thereby extended, with the resultthat accumulation of solids is promoted. In the Figure, the locationwhere accumulation of solids is promoted is indicated by the broken linesymbols.

[0128] In this way, since the gas flows out downstream and through theapertures 96 of thin plate 94, the entire thin plate 94 is effectivelyutilized for accumulation of solids. That is, the locations ofaccumulation of solids are dispersed along the entire flow pathcorresponding to the positions of apertures 96 of thin plate 94.Although the flow path cross-sectional area diminishes as accumulationof solids progresses accompanying the period of use of the device, theperiod of use of the device is extended, since local accumulation suchas occurred previously cannot occur.

[0129] A thin plate 94 a could be used bent into irregular shape orundulating shape instead of the thin plate 94 in FIG. 8. FIG. 9 is aside view showing a modified example of the trap device of the thirdembodiment. For example, the cross section of the thin plate 94 a may bebent into connected V shapes, connected U shapes or sine wave shapes etcas shown in FIG. 9. Furthermore, an irregular construction may be formedin the surface of thin plate 94 a. For example, the surface of thinplate 94 a may be formed as an irregular surface by performing blastprocessing of the surface of thin plate 94 a. If the surface area ofthin plate 94 a is increased in this way, the effective area wheresolids can be accumulated is increased, and the flow path of the gas isextended. The period of use of the device can therefore be even furtherextended, and the efficiency of collection of solids can be furtherimproved.

[0130] Fourth Embodiment

[0131] Next, a trap device according to a fourth embodiment will bedescribed. FIG. 10 is a view showing the construction of a trap deviceaccording to the fourth embodiment. FIG. 10(A) shows a perspective viewof the trap device and FIG. 10(B) shows a side view of the trap device.The structural components which are the same as those described in thethird embodiment are given the same reference numerals in FIG. 10.

[0132] The trap device of the fourth embodiment comprises acylindrically shaped vessel (casing) 86 having apertures at both ends.One aperture of this vessel 86 is employed as a gas inlet port 88 andthe other aperture of this vessel 86 is employed as a gas outlet port90. This gas inlet port 88 and gas outlet port 90 are respectivelyconnected to the exhaust path. Also, a gas flow path connected to theexhaust path mentioned above is formed within vessel 86 between gasinlet port 88 and gas outlet port 90.

[0133] In the trap device of this embodiment, the gas flow velocity inthe flow path referred to above is controlled to a certain flow velocitydependent on the position of this flow path. To achieve this, in thisfourth embodiment, the flow path referred to above is constituted by aplurality of annular first flow paths and second flow paths connectedbetween each of the first flow paths. Also, the flow pathcross-sectional area of these first flow paths is changed at prescribedpositions. In the fourth embodiment, the first flow paths referred toabove are formed by a plurality of thin plates 110. These thin plates110 are connected to the surface of cylindrically shaped shaft element92 provided in the interior of vessel 86. Also, the second flow pathreferred to above is formed by apertures 112 formed in prescribedpositions of thin plates 110 (not shown in FIG. 10(B)). Furthermore, theflow path cross-sectional area of the first flow paths is changed byforming steps 114 at prescribed positions of thin plates 110.

[0134] Shaft element 92 referred to above was described in the thirdembodiment, so the description is not duplicated here. Also, in theinterior of shaft element 92, there is provided a cooling mechanismwhich is the same as that described with reference to FIG. 7 in respectof the third embodiment. Also, at both ends of shaft element 92, thereare respectively formed apertures 98 and 100 as described with referenceto the third embodiment. Aperture 100 is not shown in FIG. 10.

[0135] Each of the thin plates 110 described above are plates extendingin annular fashion centered on shaft element 92. Thin plates 110 arearranged along shaft element 92 from the side of gas inlet port 88 tothe side of gas outlet port 90. The spaces between adjacent thin plates110 are used as the first flow path described above.

[0136] Also, as described above, a second flow path is formed byapertures 112 formed at prescribed positions of thin plates 110. Thatis, this second flow path is branched from part of the first flow pathand constitutes a flow path connected to another first flow pathadjacent to the first-mentioned first flow path. Consequently, gas thatis introduced into vessel 86 from gas inlet port 88 flows towards thegas outlet port 90 and flows along second flow paths through the firstflow paths.

[0137] Also, a step 114 is formed in thin plate 100 by bending at leastpart of thin plate 110. The size of the gap between the adjacent thinplates 110 changes at the location where such a step 114 is formed.Consequently, the flow path cross-sectional area of the first flow pathchanges in the region where the step 114 is formed. The conductance ofthe first flow path changes at the region of the step 114 i.e. thevelocity of gas flow changes.

[0138] Next, operation of the trap device constructed as described abovewill be described with reference to FIG. 11. FIG. 11 is a view providedin explanation of the operation of the trap device of the fourthembodiment. FIG. 11 shows the flow paths within vessel 86 describedabove. The two line sections extending in the horizontal direction inthe upper and lower part of the Figure indicate vessel 86; thin plate110 is indicated by the plurality of line sections arranged parallelwith each other between these line sections. Other structural componentssuch as shaft element 92 are omitted from the drawing. The left side inthe Figure is the gas inlet port 88 side; the right side in the Figureis the gas outlet port 90 side. In addition, the arrow symbols in thedrawing indicate the direction of flow of the gas.

[0139] Gas that is introduced into vessel 86 passes along the flow pathsi.e. the first flow paths and second flow paths and flows from the gasinlet port 88 side towards the gas outlet port 90 side i.e. from theleft-hand side in the Figure towards the right-hand side in the Figure.Gas that has flowed out to the first flow path from the second flow pathis branched into two streams so that, for example, the gas flows inmutually opposite directions indicated by symbols a and b in the Figure.The flow path cross-sectional area of the first flow path changes at thelocation of step 114 of thin plate 110. Where the flow pathcross-sectional area is smaller, the gas flow velocity is faster, so itis more difficult for accumulation of solids to proceed. Consequently,in these regions, reduction of conductance is prevented. On the otherhand, where the flow path cross-sectional area is larger, the gas flowvelocity is lower, so accumulation of solids is promoted. The locationswhere accumulation of solids is promoted are indicated in the Figure bythe broken line symbols.

[0140] In this way, since the gas flows out downstream and through theapertures 112 of thin plate 110, the entire thin plate 110 iseffectively utilized for accumulation of solids. That is, the locationsof accumulation of solids are dispersed along the entire flow pathcorresponding to the positions of steps 114 of thin plate 110. Althoughthe flow path cross-sectional area diminishes as accumulation of solidsprogresses accompanying the period of use of the device, the period ofuse of the device is extended, since local accumulation such as occurredpreviously cannot occur.

[0141] A thin plate 110 a could be used bent into irregular shape orundulating shape instead of the thin plate 110 in FIG. 10. FIG. 12 is aside view showing a modified example of the trap device of the fourthembodiment. For example, the cross section of the thin plate 110 a maybe bent into connected V shapes, connected U shapes or sine wave shapesetc as shown in FIG. 12. Furthermore, an irregular construction may beformed in the surface of thin plate 110 a. For example, the surface ofthin plate 110 a may be formed as an irregular surface by performingblast processing of the surface of thin plate 110 a. If the surface areaof thin plate 110 a is increased in this way, the effective area wheresolids can be accumulated is increased, and the flow path of the gas isextended. The period of use of the device can therefore be even furtherextended, and the efficiency of collection of solids can be furtherimproved.

[0142] With an exhaust gas filtration device according to the presentinvention, there is further provided an auxiliary filtration devicearranged in the exhaust path between the trap device and the filter.

[0143] Thanks to the provision of such an auxiliary filtration device,some of the solids which were difficult to collect and accumulate by thetrap device can be removed by this auxiliary filtration device upstreamof the filter. Consequently, early blockage of the filter can beprevented, and the life of the exhaust gas filtration device as a wholecan be extended.

[0144] Also, if an auxiliary filtration device is provided, theconductance of the exhaust path can be lowered to an extent such thatthe vacuum pump is not damaged. As a result, the flow velocity ofexhaust gas in the trap device is lowered i.e. the dwell time of theexhaust gas in the trap device is extended, thereby improving theefficiency of collection of solidification constituents and solids inthe trap device.

[0145] Also, with the auxiliary filtration device of the presentinvention, a filter element is arranged in the through-flow path of theexhaust gas, enabling solids in the exhaust gas to be removed by thisfilter element. Also, the conductance of the position of the exhaustpath where the filter element is arranged is lowered due to theprovision of a filter element. The flow velocity of the exhaust gasflowing through the exhaust path upstream of the position where theconductance was lowered is therefore reduced. As a result, thecollection efficiency of solidification constituents and solids in theother filtration device provided upstream of this auxiliary filtrationdevice is increased.

[0146] Also, with the trap device of the present invention, the trapdevice is constituted by a vessel having in its interior a gas flow pathconnected to the exhaust path, and the gas flow velocity in the flowpath is controlled to a prescribed flow rate in accordance with thisflow path position.

[0147] Since the gas flow velocity is thus controlled in accordance withflow path position, accumulation of solids is promoted at locationswhere the flow velocity is comparatively small. On the other hand,accumulation of solids is avoided at locations where the flow velocityis comparatively large. Consequently, it is possible to causeaccumulation of solids in prescribed positions of the flow path and toensure that solids are not accumulated in locations where it is notdesired to lower the conductance of the flow path. It is therebypossible to promote accumulation of solids in locations other than thevicinity of the gas inlet port, thereby extending the period of use ofthe device and also improving the collection efficiency of solids.

[0148] If the trap device of the present invention is applied to aplasma CVD device for example for semiconductor manufacture, theoperating efficiency and productivity of this CVD device can beimproved.

What is claimed is:
 1. An exhaust gas filtration device comprising atrap device and a filter which are arranged successively in an exhaustpath of an airtight vessel evacuated by a vacuum pump, for removingsolidification constituents and solids in the exhaust gas evacuated intothe exhaust path, further comprising: an auxiliary filtration devicearranged in the exhaust path between said trap device and the filter. 2.An auxiliary filtration device arranged in the exhaust path of anairtight vessel evacuated by a vacuum pump, for removing solids inexhaust gas evacuated into the exhaust path, comprising a vessel, anexhaust gas inlet pipe, an exhaust gas outlet pipe and a filter element:wherein this filter element is a sponge-like aggregate constituted bycollecting a large number of strip-shaped or filamentous members;wherein the interior of said vessel and said exhaust path are connectedby said exhaust gas inlet pipe and exhaust gas outlet pipe; wherein afiltration region, a first diffusion region and second diffusion regionare defined in the interior of said vessel, the first and seconddiffusion regions being separated from each other by said filter elementarranged in said filtration region; wherein the end of said exhaust gasinlet pipe constituting a gas inlet port is arranged in either saidfirst or second diffusion region, and the end of said exhaust gas outletpipe constituting a gas outlet port is arranged in either said first orsecond diffusion region; and wherein the exhaust gas flow path extendingfrom said gas inlet port to said gas outlet port is constructed suchthat exhaust gas passes through said filter element at least once.
 3. Anauxiliary filtration device according to claim 2, wherein part of saidexhaust gas flow path constitutes a path in which exhaust gas flows inthe opposite direction to the direction in which exhaust gas isevacuated from said airtight vessel.
 4. An auxiliary filtration deviceaccording to claim 2: wherein said gas inlet port is arranged in saidfirst diffusion region and said gas outlet port is arranged in saidsecond diffusion region; wherein said exhaust gas inlet pipe is coupledto said exhaust path through a partition on the side of said seconddiffusion region of said vessel; and wherein said exhaust gas outletpipe being coupled to said exhaust path through a partition on the sideof said first diffusion region of said vessel.
 5. An auxiliaryfiltration device according to claim 2: wherein a partition that dividessaid first diffusion region into an exhaust gas inlet region and anexhaust gas outlet region, and said filtration region into first andsecond filtration regions is provided in said vessel; wherein said gasinlet port is arranged in said exhaust gas inlet region and said gasoutlet port is arranged in said exhaust gas outlet region; said exhaustgas inlet pipe is coupled with said exhaust path through a partition onthe side of said second diffusion region of said vessel; and saidexhaust gas outlet pipe is coupled with said exhaust path through apartition on the side of said first diffusion region of said vessel. 6.An auxiliary filtration device according to claim 2, wherein said filterelement is constituted by a plurality of metal strips which are packedsubstantially uniformly between a plurality of support plates having atleast one aperture.
 7. An auxiliary filtration device according to claim2, wherein a cooling mechanism is provided for cooling said filterelement.